Motoko doesn’t support ::
in method names:
Identifiers are alpha-numeric, start with a letter and may contain underscores:
<id> ::= Letter (Letter | Digit | _)*
Letter ::= A..Z | a..z
Digit ::= 0..9
If we want to go with a prefix/suffix mechanism and be compatible with Motoko, then using an underscore is pretty much the only choice.